The present invention relates to sustained release herbicides. More specifically, thiocarbamate-based preemergence herbicides are contained within substantially dry, solid polymer matrices. The matrices are selected from substituted cellulosic and/or polypropylene materials which allow slow diffusion release of the herbicides, i.e., over a period of about four months. Accordingly, the compositions herein are herbicidally effective during this period and are specifically designed to provide a herbicidal effect throughout an average 31/2 to 4 month growing season.
Much effort has been expended to provide herbicidal compositions which are effective throughout a growing season, and it would be difficult to overstate the economic value of a truly effective, season-long, sustained release herbicide. For example, the user of such a product would not have to cultivate crops throughout the entire growing season. The savings in time and labor would be substantial. Moreover, crops could be planted in narrower rows since cultivator machinery would not have to be used during the growing season. This would allow more productive use of the land.
Herbicidal chemicals are, of course, designed to prevent the germination and/or growth of weeds and weed seedlings. However, at high application rates herbicides also tend to destroy desirable crops. Accordingly, it is not possible to apply common herbicides at the extremely high levels which are required to provide season-long weed control without damaging crops. Repeated applications of herbicides at weed-controlling levels (which are safe for crops) throughout the growing season is one method of crop cultivation. A more desirable method would be to use sustained release compositions which provide a metered dose of herbicide at or somewhat above its minimum effective level throughout the season.
As is well recognized in the agricultural arts, good, sustained release, season-long herbicides have not been available heretofore. In particular, no good season-long herbicide has been available for use on soybeans and like crops.
The present invention employs thiocarbamate-based pre-emergence herbicides which are safe for use on crops such as soybeans. The herbicides are contained within a polymer matrix which is specially selected to release the herbicide at an extremely slow rate, thereby providing season-long weed control.
Surprisingly, it has been discovered that many common pre-emergence herbicides are not suitable for use in the practice of this invention. Moreover, many common polymers are not useful herein. However, the particular combinations of thiocarbamate-based herbicides and selected polymer carrier matrices disclosed herein do meet the requirements for season-long weed control.